A Pakistani military source told NBC: "They burnt a teacher in front of the students in a classroom.

"They literally set the teacher on fire with gasoline and made the kids watch."

Another attacker reportedly detonated his suicide bomb in a room containing 60 children.

FIGHT: The Pakistani military fought desperately to free the hostages [BARCROFT]

“They literally set the teacher on fire with gasoline and made the kids watch”

A Pakistan military source

Prime Minister David Cameron was quick to condemn the act of terror, tweeting: "The news from Pakistan is deeply shocking.

"It's horrifying that children are being killed simply for going to school."

The Taliban claimed responsibility for the slaughter, announcing it was revenge for Pakistani military strikes against them.

A spokesman said: "We targeted the school because the army targets out families.

"We want them to feel our pain."

CONDEMNED: David Cameron described the attack as horrifying [PA]

Teen campaigner Mala Yousafzai, who was shot in the head by the Taliban in 2012, described the attack as "atrocious and cowardly".

She said: "I am heartbroken by this senseless and cold-blooded act of terror in Peshawar that is unfolding before us.

"Innocent children in their school have no place in horror such as this.

"I condemn these atrocious and cowardly acts and stand united with the government and armed forces of Pakistan whose efforts so far to address this horrific event are commendable.

HEARTBROKEN: Malala Yousafzai also spoke of her sadness following the act of terror [PA]

"I, along with millions of others around the world, mourn these children, my brothers and sisters – but we will never be defeated."

Heartbreaking images have shown wounded children wearing green school uniforms being carried into hospitals.

US president Barack Obama said: "By targeting students and teachers in this heinous attack, terrorists have once again shown their depravity.

"We stand with the people of Pakistan, and reiterate the commitment of the United States to support the Government of Pakistan in its efforts to combat terrorism and extremism and to promote peace and stability in the region."